1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system and, more particularly, to an MRI system for acquiring spin density image data in an object to be examined at high speed.
2. Description of the Related Art
As is well known, magnetic resonance imaging (MRI) is a technique of visualizing microscopic chemical/physical data of a substance by utilizing a phenomenon in which when atomic nuclei having specific magnetic moments are placed in a uniform static field, they resonantly absorb energy of a magnetic field which is rotated at a specific frequency, i.e., an RF magnetic field.
In an MRI system for obtaining an image by this MRI technique, a very long data acquisition time is required as compared with other medical imaging system such as an ultrasound imaging system and an X-ray computed tomography (CT) system. Thus, artifacts may appear or blurring may occur due to the movement of an object arising from respiration and the like. Therefore, moving portions, i.e., a cardiovascular systems are difficult to be imaged. In addition, scanning (imaging) takes a long data acquisition time, a patient inevitably experiences discomfort.
Therefore, as high-speed imaging methods of obtaining an MR image at high speed, the echo planar imaging method by Mansfield, the ultra high speed method by Hutchison et al., and the like have been proposed.
FIGS. 1A-1D show pulse sequences for image data acquisition by the echo planar imaging method.
(1) While a slicing gradient field Gs is applied as seen in FIG. 1B, a 90.degree. selective excitation RF pulse, as seen in FIG. 1A, is applied as an RF magnetic field RF so as to selectively excite magnetization at a slice plane.
(2) A 180.degree. RF pulse is applied as seen in FIG. 1A.
(3) A phase encoding gradient field Ge, as seen in FIG. 1D, is statically applied in a direction parallel to a slice plane. At the same time, a read-out gradient field Gr, as seen in FIG. 1C, is applied in a direction perpendicular to the gradient fields Gs and Ge in such a manner that the gradient field Gr is repeatedly switched to the positive and negative polarities at high speed.
FIGS. 2A-2D show pulse sequences of the ultra high speed Fourier method (also called the multiple echo Fourier method). This method is different from the echo planar imaging method in that a phase encoding gradient field Ge, as seen in FIG. 2D, is applied in the form of a pulse every time switching of a read gradient field Gr, as seen in FIG. 2C, is performed.
By using these pulse sequences, since there is a timing when phases of magnetization in a slice plane are matched with each other upon every switching of a read gradient field, magnetic resonance (MR) multiple echo trains are observed. These echo trains can be acquired as MR data required for imaging of the slice plane within a period of time in which magnetization in the slice plane which is excited by a 90.degree. pulse is relaxed due to a transverse magnetization relaxation phenomenon, and hence high-speed imaging can be performed.
However, these conventional ultra high speed imaging methods have the following problems.
In order to acquire MR data required for image reconstruction at high speed and at high efficiency, the read-out gradient Gr is switched to the positive and negative phases at high speed. Mismatching may be caused between data acquired at positive and negative time phases of the gradient field Gr by various factors, e.g., a static field inhomogeneity, a static field offset, a water/fat mixture system, detection frequency deviation, inclined data sampling in a Fourier space of a density distribution, spatial nonlinearity of gradient fields, and temporal variation of gradient fields. If such mismatching is caused, a conspicuous artifact appears on an image. This leads to a serious problem especially when a static field inhomogeneity is increased and the above-described pulse sequences are applied to a living body in which water and fat are both contained.
In addition, in the above-described echo planar method and ultra high speed Fourier method, since the strength of the phase encoding gradient Ge is very low, a phase encoding error tends to occur due to a static field inhomogeneity. Such a phase encoding error causes positional errors, changes in density data, and blurring in a reconstructed MR image.